A vaccine will be required to control the ongoing epidemic of genital herpes. Most severe, recurrent genital herpes is due to herpes simplex virus type 2 (HSV-2). Current vaccines for HSV-2 have failed or had partial activity in selected patient populations. Cellular immune responses to (HSV-2) are important in controlling infection. The induction of cellular immunity, both CD8 CTL capable of killing virally infected cells and/or halting replication,- and CD4 helper cells capable of supporting CD8 and antibody effectors, is likely to be a characteristic of a successful vaccine. The goal of this proposal is to identify targets of the cellular immune system to assist in the rational design of a vaccine. The first Aim is to identify the HSV-2 antigens-and epitopes recognized by CD8 CTL, and to place these responses in a hierarchy of immunodominance with regards to population prevalence and effector cell number. A novel expression cloning method is used for antigen discovery. To study immune responses associated with effective immunity, subjects will be carefully selected to have extremely mild HSV-2 infection as measured objectively by genital shedding rates as well as by clinical history. To reduce analytical complexity, some assays will focus on the commonest human HLA type. The second Aim is to determine if quantitative or qualitative difference in HSV-specific CD8 responses can be detected between groups of carefully characterized subjects with mild or severe chronic HSV-2 infection. Antigen- and epitope-specific assays will be applied to blood samples. The third Aim is to determine the prevalence and magnitude of CD4 helper responses to the HSV-2 antigens that are identified in Aims 1 and 2 as potentially immunodominant and/or associated with mild infection. Overall CD4 responses to HSV-2 will also be compared between the subject groups with defined HSV-2 disease severity. The results may assist in rational design of a HSV-2 vaccine or the monitoring of HSV-2 vaccine trials or immune responses modifiers that are active for recurrent HSV-2. The data will also reveal the in vivo effects of HSVencoded immune evasion genes on the CD8 CTL response within the natural host.